1. Technical Field
The present invention relates to an organic electro-luminescence (EL) device, a driving method thereof and an electronic apparatus.
2. Related Art
In recent years, increasing attention is being paid to organic EL devices employing organic EL elements as a self-luminous element requiring no backlight or the like. The organic EL display includes an organic EL layer, i.e., an emission layer between a pair of opposed electrodes. The organic EL element for full-color displaying includes emission layers that each have an emission wavelength band corresponding to a respective one of colors of red (R), green (G) and blue (B). Voltage application across the opposed electrodes induces recombination of injected electrons and holes in the emission layer, which leads to light emission by the light emitting element. The light emitting elements in the organic EL device are typically formed of thin films having a film thickness smaller than 1 μm. In addition, the organic EL display needs no backlight, which is used in liquid crystal displays, since the light emitting element itself emits light. The organic EL device therefore has an advantage of allowing an extremely small thickness thereof.
When the organic EL device is DC-driven by application thereto of a bias of one direction, impurity ions diffuse in the emission layer and are accumulated in a certain part. The accumulated impurity ions trap holes or electrons injected from the electrode, which problematically lowers the emission life and luminance. In order to address this problem, e.g. JP-A-9-293588 and JP-A-2004-114506 disclose an AC driving technique in which a forward bias voltage and a reverse bias voltage, which has the polarity opposite to that of the forward bias voltage, are alternately applied to the light emitting elements as a drive voltage for causing the light emitting elements to emit light. Since this AC driving alternately applies the voltages having the opposite polarities to the light emitting elements, the accumulation of charges and impurity ions in the light emitting elements and an internal electric field generated due to the impurity ions are alleviated. Thus, lowering of the emission life and luminance of the light emitting elements can be suppressed.
In cathode ray tube (CRT) displays, which are generally used as a display, peak-luminance displaying is performed in which the luminance of a luminescent area is enhanced if the ratio of the luminescent area to the entire display area is small. When an image of e.g. fireworks is displayed, black is displayed in almost the entire display area while displaying with an enhanced luminance is implemented in the small area corresponding to brilliance of the fireworks. Thus, the displayed image is allowed to have shape contrast. JP-A-2002-297097 discloses a technique in which an organic EL device implements peak-luminance displaying by changing voltages applied to organic EL elements depending on the ratio of a luminescent area to the entire display area.
However, in the above-described AC driving of light emitting elements, the light emitting elements emit light only when a positive voltage is applied to the anode thereof and a negative voltage is applied to the cathode thereof, i.e., only when a forward bias is applied to the light emitting elements since the light emitting elements typically have a multi-layered structure formed of an anode, an emission layer, and a cathode. That is, the light emitting elements do not emit light when a reverse bias is applied thereto in the AC driving. Therefore, the effective emission time periods are short, which leads to a problem of low brightness of displaying.
Furthermore, when it is aimed to achieve peak-luminance displaying by changing voltages applied to organic EL elements, there arises a need to provide individual power supplies that each correspond to a certain ratio of a luminance area to the entire display area. Accordingly, the size of an organic EL device is problematically increased. In contrast, a configuration employing a single variable power supply is also possible. However, use of such a power supply causes a problem that it is difficult to implement gray-scale control when the ratio of a luminescent area to the entire display area is large and therefore the drive voltage is low.